Epoxidized polyamide wet strength resins containing lecithin

ABSTRACT

A composition comprising epoxidized polyamide wet strength resin and lecithin. The composition provides wet strength to paper and molded pulp products and at the same time increases the internal bonding of the paper or molded pulp products.

This is a divisional of copending application Ser. No. 411,704 filed on9/25/89, now U.S. Pat. No. 4,970,250.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to epoxidized polyamide wet strength resinscontaining lecithin and their use in paper and molded pulp products madeof cellulose fibers such as wood pulp.

2. Background and the Prior Art

In the manufacture of wet strength paper and molded pulp products, a wetstrength resin is added to the pulp slurry. Wet strength resins aretypically of the epoxidized polyamide, urea formaldehyde or melamineformaldehyde types. These resins provide cross-linking to impart wetstrength required by various paper and molded pulp products.

During the paper or molded pulp drying process, polymers such as thoseof melamine formaldehyde and urea formaldehyde may become a healthhazard due to release of formaldehyde. Also, the epoxidized polyamidepolymers as well as the melamine formaldehyde and urea formaldehydepolymers can at times stick to hot dryer surfaces. This problem isparticularly acute with the epoxidized polyamide wet strength resins inthe making of molded pulp products. Thus, in the manufacture of moldedpulp products, wherein melamine formaldehyde wet strength resin is used,we have found that sticking is not a problem. However, the use ofepoxidized polyamide in place of the melamine formaldehyde causedundesirable sticking of resin and pulp to the drier and furthermore theinternal bond strength of the dried molded pulp product was weak. Also,it appears that urea formaldehyde wet strength resins are also not assusceptible to sticking to heated drier surfaces as with the epoxidizedpolyamides.

The application of various release agents to paper making dryer surfacesas well as to heated platens in pressing glue coated wood particles tomake panels is well known for preventing sticking of resin to suchsurfaces. However, such application of a surface lubricant means theaddition of another process step with the consequent increase inproduction time as well as an additional cost due to the amount oflubricant needed. Also, for release of molded pulp products from molds,additional difficulties are encountered in application of release agentsdue to the contoured and curvilinear surfaces of such molds.

In the making of wood based panels such as particleboard, by usingmelamine formaldehyde glues, press operators have applied an emulsion offive parts of lecithin in four parts aqua ammonia of 26 Baume and 91parts of water as a release agent on the press surfaces. Such releaseagent is sold by Borden, Inc. under the designator PC-803L.

U.S. Pat. No. 4,076,896 of Feb. 28, 1978, shows the manufacture oflaminates by impregnating paper with a melamine formaldehyde gluecontaining lecithin wherein the lecithin increases the releasecharacteristics of the resin when pressing out a panel. Japanese patentpublication JP55-139430 to Matsushita Elec. Works relates to themanufacture of a laminated sheet which includes impregnating paper orcloth with a thermosetting resin containing lecithin wherein the resinsare said to include phenol resins, epoxy resins, polyester resins andmelamine resins. U.S. Pat. No. 4,267,240 of May 12, 1981 to FormicaCorp. relates to a release sheet comprising a web of paper having oneside coated with various materials including lecithin.

The Kamikaseta et al U.S. Pat. No. 4,634,727 of Jan. 6, 1987 relates toa polyvinyl acetate emulsion adhesive for bonding wood and other poroussubstances wherein lecithin is added directly to the adhesive emulsionor the lecithin is first emulsified with aqueous ammonia before additionto the adhesive. In this 727 patent, the lecithin is said to assist inrelease of the adhesive from the press platens and increases the bondingstrength of the adhesive. The 727 patent states that various additionalpolymers may be added to the lecithin containing polyvinylacetate suchas urea resin, phenol formaldehyde resin and melamine resin.

Japanese patent publication, JP 1045894 to OJI Paper KK relates to themanufacture of paper which is said to have improved releasability byhaving release agents added to a layer of paper wherein the releaseagents include lecithin. Japanese Patent publication JP 88-057206 toKobe relates to the production of a laminate by preimpregnating a papersubstrate with a solution containing a surfactant and a cure acceleratorfor a phenolic resin varnish wherein lecithin is referred to as a cureaccelerator. U.S. Pat. Nos. 1,977,251 to Stallmann of Oct. 16, 1934 and3,947,383 to Baggett of Mar. 30, 1976 describe reaction products ofammonia and epichlorohydrin for use as a paper wet strength resinadditives.

SUMMARY OF THE INVENTION

We have now found that the addition of lecithin in epoxidized polyamidewet strength resins eliminates the sticking problem encountered on theheated driers in the manufacture of paper and particularly in themanufacture of molded pulp products. The lecithin is preferablydispersed in an emulsifying or dispersing agent prior to itsincorporation in the epoxidized polyamide. The epoxidized polyamidecontaining lecithin is added to the pulp slurry prior to forming of themolded product or paper on the wire mesh. Alternatively, each of theepoxidized polyamide and lecithin can be added separately to the aqueouspulp slurry. The addition of lecithin also improves the internal bondingof pulp in paper and molded pulp products which utilize epoxidizedpolyamide wet strength resins.

DESCRIPTION OF THE INVENTION

The epoxidized polyamide wet strength resins are water soluble cationicthermosetting resins. They are generally sold as aqueous solutionscontaining from about 10% to 35% by weight of resin solids, i.e., about10% to 35% by weight of the epoxidized polyamide. Curing of suchpolyamides in paper and molded pulp products on hot drying surfacesincreases the wet strength of the paper or molded pulp product.Generally, sufficient wet strength resin is added so that the wetstrength of the paper or molded pulp product is greater than about 15percent of its dry strength. Wet strength is the load required to breakthe paper when completely wet with water. The strength measurements mayinclude wet tensile, wet mullens (burst), and wet tear. Paper and otherpulp products manufactured without any additives do not have wetstrength. By the term paper, we mean to include paperboards, toweling,tissue, food board, linerboard and corrugating medium. By the termmolded pulp products, we mean to also include molded pulp/textilecontaining products. Molded pulp products are contoured products made ofpulp such as egg packaging items, food trays, plates, flower pots,bottle protectors, and the like. Illustrative of molded pulp productswhich contain textiles, there can be mentioned contoured products suchas the interior part of automobile doors, panels, etc. In contouredproducts, the contour is a part of the permanent shape of the articleinvolved and such products often have surfaces which are that ofcompound curves.

Epoxidized polyamide wet strength resins are well known materials andtheir composition and method of preparation are amply described in theliterature such as in the following U.S. Pat. Nos.: 3,565,754;3,733,290; 3,793,279; 3,887,510, 3,914,155; and 4,501,862. As can beseen from the above references, amides which may be reacted withepihalohydrins, e.g., epichlorohydrin, to form the polyamide wetstrength resin are also referred to as polyaminopolyamides. Thepolyaminopolyamides are generally prepared by reacting polycarboxylicacids, or their esters with polyalkylenepolyamines such as those havingtwo primary amine groups and at least one secondary or tertiary aminegroup. The polycarboxylic acids or esters thereof can be aromatic oraliphatic. The acids are generally C₂ to C₂₀ saturated aliphaticdicarboxylic acids and the esters can be formed by reacting such acidswith alkanois having from 1 to about 4 carbon atoms. Thepolyaminopolyamides are then epoxidized to form the epoxidized polyamidewet strength resins. Some of the epoxidized polyamide wet strengthresins are modified with other reactants or the starting materials forsuch resins are modified. Thus, the polyaminopolyamide can be reactedwith other compounds such as urea or formaldehyde or the acids can besubstituted such as in the case of nitrilotriacetic acid.

A preferred class of epoxidized polyamide wet strength resins aredisclosed in U.S. Pat. No. 3,887,510 which issued to Chan et al on June3, 1975. In the Chan et al patent, dicarboxylic diesters derived from C₃to C₆ saturated aliphatic dicarboxylic acids and respectively C₁ to C₃saturated aliphatic monohydric alcohols are reacted with apolyalkylenepolyamine to prepare the polyaminopolyamide. As to the acidsfrom which the esters are derived, there can be mentioned malonic,succinic, glutaric and adipic acids. The alcohols can be singly or incombination, methanol, ethanol, n-propanol or isopropanol. Methyl esterssuch as dimethyladipate and dimethylgluterate are the preferred esters.

Illustrative of of suitable polyalkylenepolyamines of the Chan et alpatent, there can be mentioned: diethylenetriamine;triethylenetetramine; tetraethylenepentamine; dipropylenetriamine;4-methyldiethylenetriamine; 5-methyldipropylenetriamine;4,7-dialkyltriethylenetetramine; and dihexylenetriamine. Thepolyalkylenepolyamines of the Chan et al patent have the genericformula:

H₂ NC_(n) H_(2n) (NRC_(n) H_(2n))_(x) NH₂ wherein R is either C₁ to C₄alkyl or hydrogen, x can vary from 1 to about 5 and n can vary fromabout 2 to 6. In some cases however, it is desirable to increase thespacing of secondary amine groups and this can be done by substituting adiamine such as ethylenediamine, hexamethylenediamine and the like for aportion of the polyalkylenepolyamine. A preferred epoxidized polyamideis that made from the polyamide resulting from reaction of dimethylgluterate and diethylene triamine.

The incorporation of lecithin in a pulp slurry containing epoxidizedpolyamide inhibits sticking of the wet strength resin or pulp fibers tothe surfaces of the driers. Furthermore, such use of lecithin increasesthe bonding strength of the pulp to itself or through the action of wetstrength resin. The quantity of lecithin used in the aqueous pulp slurryto obtain the advantages of this invention varies over a broad rangesuch as from about 0.1 percent to 15 percent or more by weight oflecithin based on the weight of the wet strength resin solids, i.e., theepoxidized polyamide. Preferably the quantity of lecithin in the pulpslurry is from about 2 to about 10 percent by weight of lecithin basedon the weight of the resin solids.

A preferred composition of this invention is a stable concentrate of thewet strength resin and lecithin which can be added to the aqueous pulpslurry. Such composition contains at least 61 percent of water andcomprises an aqueous solution of epoxidized polyamide wet strength resinhaving a solids content of from about 8 to 35 percent by weight of saidcomposition, from about 0.1 percent to 12 percent by weight of lecithinbased on the weight of said resin solids and from about 61 percent toabout 92 percent by weight of water and wherein said composition has apH of about 3 to 5. Preferably, such composition contains from about 10percent to 30 percent by weight of said resin solids and 6 to 10 percentlecithin, based on said resin solids and 67 to about 89 percent ofwater. These concentrates can also contain small quantities, e.g., up toabout 2 or 5% by weight based on the weight of resin solids, of varioussolvents, emulsifiers or dispersing agents for the lecithin.

The lecithin can be added to the aqueous pulp slurry directly or it canfirst be added to the wet strength resin which is subsequently added tothe aqueous pulp slurry. Preferably the lecithin is incorporated in thewet strength resin, as the above described concentrate, before additionof these chemicals to the pulp slurry. Lecithin is not soluble in water.Therefor it is preferred that a solution, emulsion or dispersion of thelecithin be used. Alternatively, hydrolized lecithin can be employed orthe lecithin itself can be intimately dispersed into the wet strengthresin or pulp slurry such as by mixing. A preferred way for getting thelecithin into the wet strength resin or directly into the aqueous pulpslurry is by first emulsifying the lecithin in aqueous ammonia watersuch as an emulsion containing 5 parts lecithin, 4 parts aqua ammonia of26 Baume and 91 parts of water.

The molded pulp products and various paper products of this inventionare made by conventional techniques except that a small quantity oflecithin is used together with the epoxidized polyamide wet strengthresin in the aqueous pulp slurry in making such products.Illustratively, the molded pulp products can be made by depositing pulpfibers from a slurry on to a foraminous, e.g., wire mesh, mold. Therecan be single or multiple molds which can be fixed or as part of aconveyer so that a continuous operation can be realized. Molds on aconveyer often involve a rotating cylinder with suitable portingconnections. Wet preforms from the initial mold are often pressed to adesired thickness and then dried under restraint between matched heateddies or in an oven. The drying process will vary depending upon thedensity of the finished product but can vary from about 1 to about 3minutes. The thickness of the molded pulp products can vary over a widerange such as that of about 0.1 inch to about 0.4 inch.

Briefly, in the production of various papers, including paperboard, thepaper furnish, after stock preparation and proper dilution, is usuallysent to the paper machine through one or more screens or other devicesto remove dirt and fiber bundles. It then proceeds to a flow spreader toprovide a uniform flowing stream of the width of the machine. The flowspreader discharges the slurry into a headbox, where turbulence iscontrolled, fiber flocculation is minimized, and the proper head isprovided to cause the slurry to flow out through the slice and onto amoving wire at the proper velocity. The sheet leaving the "wet end" ispressed to remove additional water by mechanical means. At this stage,the wet sheet has reached the point where further water removal bymechanical means is not feasible and evaporative drying must beempolyed. The evaporative driers are generally steam heated cylinders,with alternate sides of the wet paper exposed to the hot surface as thesheet passes from cylinder to cylinder.

During the manufacture of paper or molded pulp products, severaladditives are introduced at the "wet end" of the process, i.e., in thepulp slurry, to give the finished products the required physicalproperties. One of these additives is the wet strength resin. Duringdrying on hot surfaces, the wet end additives, including the wetstrength resins, may cause residual build-up on heated dryer surfaces.This bulid-up will cause the molded product or paper to stick to thedryer causing dryer wrap or breaks in the case of cylinder dryers or theneed for manual removal of the contoured molded product in the case ofmolded pulp products. The use of lecithin as set forth in this inventionovercomes or minimizes these sticking or breaking problems.Additionally, the lecithin increases the strength of internal bonding inpaper and molded pulp products. The percentage of dry pulp solids in theaqueous pulp slurry vary over a wide range but, initially, prior to thedraining of water are on the order of about 0.5 to 5 percent of theslurry. The quantity of epoxidized polyamide to dry pulp solids in theslurry will generally vary from about 0.1% to about 5%.

In order that those skilled in the art may more fully understand theinventive concept presented herein, the following examples are set forthin the appended claims. All parts and percentages are by weight unlessotherwise stated.

EXAMPLE 1

Epoxidized poylamide wet strength resin, namely Cascamid C-20, was addedto a pulp slurry at a rate of 2.25% (45 pounds of wet strength resinsolids per ton of dry pulp solids) to form a 0.65 gm/cm³ molded pulpsheet. Cascamid C-20 is an aqueous solution of wet strength resin soldby Borden, Inc., having 20 parts of epoxidized polyamide in 80 parts ofwater and wherein the polyamide, prior to epoxidation is the reactionproduct of dimethylgluterate and diethylenetriamine. The sheets wereformed in a deckel box, pressed to a thickness of 2.5 mm and dried onmatching die driers at 530° F., for 2.5 minutes. Several of the sheetswere observed to stick to the upper die drier as the lower onedescended. The sheets also showed evidence of picking which wasdetermined through visual observation and roughness of test specimens.In an attempt to cut specimens for testing, the sheet broke across thecenter internally at a place intermediate its top and bottom surfacesdue to internal bond failure resulting from what was attributed toimproper cure of resin and/or moisture escaping from the center.

EXAMPLE 2

Laboratory handsheets were prepared as in Example 1 above except thatthe resin addition level was reduced to 0.5% (10 pounds of wet strengthresin solids per ton of dry pulp solids) and a contoured die disc wasnot used. Test samples were placed between heated press platens fordrying. Upon lowering the lower plate, the specimens stuck to the upperplate and again showed bonding failure through the center of thespecimen, i.e., internally in a plane intermediate its top and bottomsurfaces.

EXAMPLE 3

Laboratory handsheets were prepared as in Example 2 above except that0.1% of PC-803-L (by weight, based on the weight of resin solids inC-20) and 0.5% Cascamid C-20 (by weight, based on the weight of dry pulpsolids) were each added separately to the slurry. Handsheets did notshow signs of sticking or internal failure on cutting. PC-803-L is anaqueous emulsion of lecithin in ammonia. The aqueous emulsion consistsof 5 parts of lecithin in 4 parts of ammonia aqua of 26 Baume and 91parts of water and is sold by Borden, Inc. as a protective coating to beapplied to platen surfaces used to make particleboard with melamineformaldehyde glues.

EXAMPLE 4

Laboratory handsheets were prepared as in Example 2 above except that0.5% Cascamid C-20 (by weight, based on the weight of dry pulp solids)mixed with 0.1% of PC-103-L (by weight, based on the weight of wetstrength resin solids) before addition to the slurry. The sheets did notshow signs of sticking or internal failure. Cascamid C-20 is an aqueoussolution of wet strength resin sold by Borden, Inc. having 20 parts ofepoxidized polyamid in 80 parts of water and wherein the polyamide,prior to epoxidation is the reaction product of dimethylgluterate anddiethylenetriamine. PC-103-L is a product sold by Borden, Inc. andreferred to as a protective coating. It is composed of an aqueousemulsion of 5 parts lecithin in 4 parts of ammonia aqua of 26 Baume and91 parts of water.

EXAMPLE 5

A mixture of Cascamid C-20 (69.32%) at 20% solids, 29.64% of PC-803L and1.04% hydrochloric acid was applied to pulp/synthetic pulp slurry at0.5% (10 pounds per ton) as in 1 above showed no signs of sticking norof internal failure and had satisfactory properties with respect topercent swell, burst flexural strength and modules of elasticity. Thiswas 10.69% lecithin by weight based on weight of the resin solids.

EXAMPLE 6

Specimens were prepared as in Example 5 above except that the resin wasapplied at 1% (20 pounds of resin solids per ton of dry pulp solids) andthe PC-803L and hydrochloric acid were not used. The sheets stuck todrier surface and showed signs of internal failure.

EXAMPLE 7

Specimens were prepared as in Example 1 above except that the resinmixture was 69.32%, Cascamid C-25 at 25% solids, 29.64% of PC-803L and1.04% hydrochloric acid. Specimens showed no signs of sticking orinternal failure. The amount of lecithin in the PC-803L amounted to 8.6%of lecithin on the resin solids.

What is claimed is:
 1. An aqueous pulp slurry containing a thermosettingepoxidized polyamide wet strength resin in an amount sufficient toimpart wet strength to said pulp in the manufacture of paper and moldedpulp products and lecithin in an amount sufficient to increase theinternal bonding strength and to inhibit sticking of said polyamide onheated drying surfaces in the manufacture of paper and molded pulpproducts.
 2. An aqueous pulp slurry of claim 1 wherein the quantity ofsaid polyamide is from about 0.1 to 5 percent by weight based on theweight of dry pulp solids.
 3. A composition of claim 2 wherein thequantity of lecithin varies from about 2 percent to about 10 percent byweight based on the weight of the said polyamide resin.
 4. A process formaking paper and molded pulp products which comprises forming a mat orsheet of pulp from an aqueous pulp slurry containing thermosettingepoxidized polyamide resin in an amount sufficient to impart wetstrength to said mat or sheet upon drying of said mat or sheet on heatedsurfaces and curing of said resin, said slurry also containing lecithinin an amount sufficient to prevent said resin from sticking to saidheated surfaces and for increasing the internal bonding strength of saidpaper and molded pulp products.
 5. The process of claim 4 for makingmolded pulp products wherein said products have a permanent contour anda thickness of about 0.1 to 0.4 inches.
 6. A molded pulp productcomprising pulp containing a thermoset epoxidized polyamide wet strengthresin in an amount sufficient to impart wet strength to said product andlecithin in an amount sufficient to increase the internal bondingstrength of said pulp.
 7. A molded product of claim 6 having a thicknessof from about 0.1 to 0.4 inches.